1. Field of the Invention
The present invention relates to an accurate current generator, providing a current which is stable with respect to temperature and to the manufacturing process of the generator.
2. Discussion of the Related Art
An accurate current generator is often used in a digital-to-analog converter providing a current output depending on the generator.
FIG. 1 shows a conventional accurate current generator. This generator includes an accurate current source 10, such as a "band-gap" source, which provides a constant voltage Vbg independent from the temperature and the manufacturing process. This constant voltage Vbg is applied to the non-inverting input of an operational amplifier 12 which controls a follower transistor MN1, generally an N-channel MOS transistor. The source of transistor MN1 is connected to the inverting input of operational amplifier 12 and supplies a resistor R connected to a ground GNDe.
With this configuration, the potential of the source of transistor MN1 is set to value Vbg provided by accurate source 10. Thus, a current determined by constant voltage Vbg and resistor R settles in transistor MN1. This current forms the generator output current. The output current is generally provided, as shown, to the input of a current mirror including two P-channel MOS transistors MP1 and MP2. The sources of transistors MP1 and MP2 are connected to a high supply potential Vdd. The gates of transistors MP1 and MP2 and the drain of transistor MP1 are connected to the drain of transistor MN1. With this configuration, the output current of the generator is copied on the drain of transistor MP2 and of any other transistor connected to transistor MP1 like transistor MP2.
The stability of the current provided by the generator (according to the temperature and the manufacturing process) depends on the stability of resistor R and of voltage Vbg. Band-gap source 10 provides a particularly stable voltage Vbg. However, the integrated resistors are not very stable. Thus, resistor R is most often external and connected, as shown, between an external terminal GNDe and an integrated circuit pin. The integrated portion of the current generator, especially band-gap source 10, is connected to an internal ground GNDi. Of course, this internal ground is connected to external ground GNDe by a pin of the integrated circuit, as shown.
However, the internal ground is not directly accessible from the outside, and the connection is generally performed through the integrated circuit substrate. This substrate and its connection to external ground GNDe have an impedance Z. The current generator is most of the time integrated with digital circuits which inject noise into the substrate. This noise Vn reappears across impedance Z.
Assuming that internal ground GNDi is at potential 0, external ground GNDe will be at potential -Vn, while the source of transistor MNI, regulated with respect to internal ground GNDi, is at reference potential Vbg. Accordingly, the voltage across resistor R is equal to Vbg+Vn, whereby the output current of the generator is equal to (Vbg+Vn)/R and includes a non-negligible noise component Vn/R.
The only way to filter out this noise is to connect a capacitor, as shown in dotted lines, between the gates of transistors MP1 and MP2 and internal ground GNDi. However, the gates of transistors MP1 and MP2 are at low impedance due to the diode connection of transistor MP1, which requires a filtering capacitor of high value and difficult to reasonably integrate.
To overcome this problem, it is provided in some applications to implement resistor R in integrated form. In the current provided by the generator, the contribution of noise Vn created between the internal and external grounds is thus eliminated. However, the resistor is then highly dependent on the temperature and the manufacturing process.